Transmitting apparatus, receiving apparatus, control station, communication system, and transmission precoding method

ABSTRACT

A transmitting apparatus includes transmission antennas capable of forming a plurality of beams respectively directed to a plurality of terminals and a precoder unit that performs precoding on signals transmitted from the transmission antennas such that received power in the terminals excluding a desired terminal serving as a transmission destination of a transmission signal and IUI terminals, which are the terminals other than the desired terminal, is equal to or smaller than a threshold.

FIELD

The present invention relates to a transmitting apparatus, a receivingapparatus, a communication system, and a transmission precoding methodfor performing multi-user MIMO (Multiple-Input Multiple-Output)transmission.

BACKGROUND

In recent years, as a wireless communication system that realizeshigh-speed transmission in a limited frequency band, a multi-user MIMO(MU (Multi-User)-MIMO) system obtained by applying a space divisionmultiple access (SDMA) scheme to an MIMO system in which pluralities ofantennas are set in both of a transmitter and a receiver has beenactively examined. In the MU-MIMO system, a plurality of terminalsincluding pluralities of antennas are present with respect to a basestation including a plurality of antennas. The base station performssimultaneous transmission to the terminals in the same wirelessfrequency band.

In communication in a downlink, that is, a direction from the basestation to the terminal in the MU-MIMO system, signals aresimultaneously transmitted from the base station to the terminals.Therefore, in general, in a received signal in the terminal, signals tothe other terminals are included in addition to a desired signal, whichis a signal to the own terminal. That is, inter-user interference (IUI),which is interference caused by the signals to the other terminalsoccurs. IUI measures are desirably performed on the base station side,where limitations concerning a processing amount and the number ofantennas are small compared with the terminals, as much as possible.Therefore, in the downlink in the MU-MIMO system, the base stationcarries out processing called precoding as the IUI measures. Theprecoding indicates processing for forming a beam by weighting aplurality of signals transmitted from a plurality of antennas.

As a representative precoding method performed as the IUI measures inthe downlink in the MU-MIMO system, block diagonalization (BD) methodhas been extensively examined. Please refer to, for example, Non PatentLiteratures 1 and 2. The BD method is a precoding method for forming abeam space to direct null to terminals other than a desired terminal,that is, form directivity for reducing received power in the terminalsother than the desired terminal to 0. By applying the BD method to allthe terminals, it is possible to realize a MU-MIMO system in which theIUI does not occur. Consequently, it is possible to simplify processingand an apparatus configuration in the terminals.

CITATION LIST Non Patent Literature

-   Non Patent Literature 1: M. Rim, “Multi-user downlink beamforming    with multiple transmit and receive antennas,” Electron. Lett., vol.    38, no. 25, pp. 1725-1726, December 2002.-   Non Patent Literature 2: L. U. Choi and R. D. Murch, “A Transmit    Preprocessing Technique for Multiuser MIMO Systems Using a    Decomposition Approach,” IEEE Trans. On Wireless Commun., vol. 3,    no. 1, pp. 20-24, January 2004.

SUMMARY Technical Problem

When a plurality of transmission antennas are present, a transmissiondiversity effect is obtained. However, in the BD method, because nullsteering for directing null to the terminals other than the desiredterminal is performed, a degree of freedom of a beam formed by aplurality of signals transmitted from the antennas of the base stationis lost. Therefore, in the precoding to which the BD method is applied,it is difficult to form a beam to increase the transmission diversityeffect, that is, improve a received signal-to-nose power ratio (SNR:Signal-to-Noise power Ratio) of the terminals. In particular, in anenvironment in which a larger number of terminals are present, thedegree of freedom of the beam formation is greatly lost because of thenull steering for the terminals. In this way, in the DB method, there isa problem in that it is difficult to improve a transmission diversitygain.

The present invention has been devised in view of the above, and anobject of the present invention is to obtain a transmitting apparatusthat can improve a transmission diversity gain compared with the BDmethod.

Solution to Problem

To solve the problems and achieve the object, a transmitting apparatusaccording to the present invention includes a plurality of transmissionantennas capable of forming a plurality of beams respectively directedto a plurality of receiving apparatuses. The transmitting apparatusaccording to the present invention includes a precoder to performprecoding on signals transmitted from the transmission antennas suchthat received power in third receiving apparatuses, which are thereceiving apparatuses excluding a first receiving apparatus serving as atransmission destination of a transmission signal among the receivingapparatuses and a second receiving apparatus, which is one of thereceiving apparatuses, is equal to or smaller than a threshold.

Advantageous Effects of Invention

The transmitting apparatus according to the present invention achievesan effect that it is possible to improve a transmission diversity gaincompared with the BD method.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a configuration example of a communicationsystem according to a first embodiment.

FIG. 2 is a diagram showing a configuration example of a base station inthe first embodiment.

FIG. 3 is a diagram showing a configuration example of a terminal in thefirst embodiment.

FIG. 4 is a diagram showing a configuration example of a processingcircuit in the first embodiment.

FIG. 5 is a diagram showing a configuration example of a control circuitin the first embodiment.

FIG. 6 is a diagram representing a flowchart showing an example of aprocessing procedure in a precoder unit in the first embodiment.

FIG. 7 is a diagram showing an average SNR during precoding applicationwith respect to an average SNR during precoding non-application in thecase of the number of base station transmission antennas T=16, thenumber of terminals m=8, and the number of terminal reception branchesN_(w)=2.

FIG. 8 is a diagram representing a flowchart showing an example of aprocessing procedure of an ordering unit in the first embodiment.

FIG. 9 is a diagram showing a configuration example of a base stationnot including the ordering unit.

FIG. 10 is a diagram showing the configuration of a control station anda base station controlled by the control station in a second embodiment.

FIG. 11 is a diagram showing a configuration example of the base stationin the second embodiment.

FIG. 12 is a diagram showing a configuration example of a base stationin a third embodiment.

FIG. 13 is a diagram representing a flowchart showing an example of aprocessing procedure in a precoder unit in the third embodiment.

DESCRIPTION OF EMBODIMENTS

Transmitting apparatuses, receiving apparatuses, communication systems,and transmission precoding methods according to embodiments of thepresent invention are explained below with reference to the drawings.Note that the present invention is not limited by the embodiments.

First Embodiment

FIG. 1 is a diagram showing a configuration example of a communicationsystem according to a first embodiment of the present invention. Asshown in FIG. 1, the communication system in this embodiment includes abase station 1 and a terminal 2-1 to a terminal 2-m; m is an integerequal to or larger than 2. In the following explanation, the terminals2-1 to 2-m are sometimes referred to as users. When the terminals 2-1 to2-m are explained without being distinguished, the terminals 2-1 to 2-mare described as terminals 2. The base station 1 includes a plurality ofantennas. The terminals 2-1 to 2-m include one or more antennas.

In this embodiment, downlink communication, which is communication fromthe base station 1 to the terminals 2, is explained. Therefore, the basestation 1 is a transmitting apparatus and the terminals 2 are receivingapparatuses. In the communication system in this embodiment, a MU-MIMOscheme is used in the downlink communication. The base station 1 carriesout precoding on transmission signals transmitted from the antennas andis capable of forming beams directed to the terminals 2. Note that thebase station 1 and the terminals 2 can perform communication in whichthe terminals 2 are transmitting apparatuses and the base station 1 is areceiving apparatus, that is, uplink communication. An uplinkcommunication method can be any communication scheme.

First, terms in this embodiment are explained. In the followingexplanation, physical transmission and reception antennas are referredto as “antennas” and an array of a plurality of antennas included in oneapparatus, that is, an antenna group is referred to as “array”.Alternatively, a plurality of signal arrays corresponding to the arrayis sometimes also simply referred to as array for convenience. An arrayof a plurality of transmission antennas is referred to as “transmissionarray”. An array of a plurality of reception antennas is referred to as“reception array”. An effective number of antennas observed when thetransmission array or the reception array is multiplied with a weightmatrix, which is a matrix indicating weight, is referred to as “branch”.The number of reception branches, which are branches on a receptionside, is the number of data transmitted to the terminals 2, which arethe receiving apparatuses, in parallel and is the number of rows of areception weight matrix, which is a weight matrix multiplied in theterminals 2. The number of transmission branches, which are branches ona transmission side, is the number of rows of a transmission weightmatrix, which is a weight matrix multiplied in the base station 1, whichis the transmitting apparatus, that is, transmission precoding.

There is no limitation on the number of antennas included in theterminals 2. The present invention is also applicable when the number ofantennas is different in each of the terminals 2 and when the number ofreception branches is different in each of the terminals 2. However, tosimplify the explanation, in the following explanation, the number ofantennas included in the terminals 2 is R(R is an integer equal to orlarger than one) irrespective of the terminals. In the terminals 2,N_(w) (N_(w)≦R) weight matrixes are multiplied with the reception array.Therefore, the number of reception branches per one terminal 2 observedfrom the base station 1, which is the transmitting apparatus, is N_(w)irrespective of the terminals 2. Consequently, a total number ofreception branches N_(w, total), which is the number of branches of allthe terminals, is N_(w, total 1)=Σ_(k-1) ^(m)(N_(w))=m×N_(w). Weightapplied to the reception array is assumed in calculation of theprecoding matrix. Any weight can be applied as the weight. For example,weight in the case of N_(w)=R can be a unit matrix or can be aneigenvector matrix of a transmission line matrix. Any matrix can be usedas the reception weight matrix. Note that, in the following explanation,it is assumed that the number of antennas T of the base station 1 andthe number of reception branches N_(w) of the terminal 2 satisfy arelation of T≧N_(w,total)−N_(w)=(m−1)×N_(w).

Subsequently, the downlink communication in the communication system inthis embodiment, in which the MU-MIMO scheme is adopted, is modeled by aformula. A transmission signal vector transmitted to a terminal 2-i(i=1, . . . , and m) is represented as s(bold face)_(i)(t), atransmission power distribution matrix, which is a matrix indicatingpower distribution to the terminal 2-i, is represented as P(boldface)_(i), and a precoding matrix, that is, a beam formation matrixcorresponding to the terminal 2-i is represented as B(bold face)_(i). Atrue transmission line matrix of R×T from an antenna of the base station1 to an antenna of the terminal 2-i is represented as H(boldface)(hat)_(i), a reception weight matrix of Nw×R of the terminal 2-i isrepresented as W(bold face)_(i), and a true received signal vectorbefore reception weight multiplication of the terminal 2-i isrepresented as y(bold face)_(i)(t). Further, a received signal vectorafter the reception weight multiplication of the terminal 2-i isrepresented as r(bold face)_(i)(t). A true reception thermal noisevector in the transmission line from the antenna of the base station 1to the antenna of the terminal 2-i is represented as n(boldface)(hat)_(i)(t). At this point, a system model obtained by modelingthe communication system in this embodiment according to a formula canbe defined by the following Expression (1).

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack & \; \\{\begin{bmatrix}{r_{1}(t)} \\\vdots \\{r_{m}(t)}\end{bmatrix} = {{\begin{bmatrix}W_{1} & \ldots & O \\\vdots & \ddots & \vdots \\O & \ldots & W_{m}\end{bmatrix}\begin{bmatrix}{y_{1}(t)} \\\vdots \\{y_{m}(t)}\end{bmatrix}} = {\begin{bmatrix}W_{1} & \ldots & O \\\vdots & \ddots & \vdots \\O & \ldots & W_{m}\end{bmatrix}\left( {{{\begin{bmatrix}{\hat{H}}_{1} \\\vdots \\{\hat{H}}_{m}\end{bmatrix}\begin{bmatrix}B_{1} & \ldots & B_{m}\end{bmatrix}}\begin{bmatrix}\sqrt{P_{1}} & \ldots & O \\\vdots & \ddots & \vdots \\O & \ldots & \sqrt{P_{m}}\end{bmatrix}}\left. \quad{\begin{bmatrix}{s_{1}(t)} \\\vdots \\{s_{m}(t)}\end{bmatrix} + \begin{bmatrix}{{\hat{n}}_{1}(t)} \\\vdots \\{{\hat{n}}_{m}(t)}\end{bmatrix}} \right)} \right.}}} & (1)\end{matrix}$

Further, N_(w)×T matrix obtained by multiplying together the receptionweight matrix W(bold face)_(i) and the true transmission line matrixH(bold face)(hat)_(i) is represented as a new transmission line matrixH(bold face)_(i) and an N_(w)-th order vector obtained by multiplyingtogether the true reception thermal noise vector n(boldface)(hat)_(i)(t) and the reception weight matrix W(bold face)_(i) isrepresented as a new reception thermal noise vector n_(i)(bold face)(t).Then, the system model can be represented by the following Expression(2).

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack & \; \\{\begin{bmatrix}{r_{1}(t)} \\\vdots \\{r_{m}(t)}\end{bmatrix} = {{{\begin{bmatrix}H_{1} \\\vdots \\H_{m}\end{bmatrix}\begin{bmatrix}B_{1} & \ldots & B_{m}\end{bmatrix}}\begin{bmatrix}\sqrt{P_{1}} & \ldots & O \\\vdots & \ddots & \vdots \\O & \ldots & \sqrt{P_{m}}\end{bmatrix}}{\quad{\begin{bmatrix}{s_{1}(t)} \\\vdots \\{s_{m}(t)}\end{bmatrix} + \begin{bmatrix}{n_{1}(t)} \\\vdots \\{n_{m}(t)}\end{bmatrix}}}}} & (2)\end{matrix}$

The above Expression (2) can be represented as the following Expression(3).

[Math. 3]

r (t)= HBPs (t)+ n (t)  (3)

In the expression, H(bold face)(bar) is a system transmission linematrix of N_(w, total)×T indicating a transmission line from the antennaof the base station 1 to all branches of all the terminals 2 after themultiplication of the reception weight. B(bold face)(bar) is a systemprecoding matrix of T×N_(st) for all the terminals 2 in the base station1. Note that N_(st) is a total number of signals simultaneouslytransmitted to all the terminals 2 in parallel. P(bold face)(bar) is asystem transmission power matrix, which is a matrix that setstransmission power distribution to all the terminals 2, s(boldface)(bar) (t) is an N_(st)-th order system transmission vectorindicating transmission signals to all the terminals 2, and n(boldface)(bar) (t) is an N_(w, total)-th order system noise vector, which isa noise vector for all the terminals 2 after the reception weightmultiplication. As indicated by the following Expression (4), a productof H(bold face)(bar) and B(boldface)(bar) can be grasped as an effectivesystem transmission line matrix H(bold face)(bar)_(e) by transmissionbeam formation.

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 4} \right\rbrack & \; \\{{\overset{\_}{H}}_{e} = \begin{bmatrix}{H_{1}B_{1}} & {H_{1}B_{2}} & \ldots & {H_{1}B_{m}} \\{H_{2}B_{1}} & {H_{2}B_{2}} & \ldots & {H_{2}B_{m}} \\\vdots & \vdots & \ddots & \vdots \\{H_{m}B_{1}} & {H_{m}B_{2}} & \ldots & {H_{m}B_{m}}\end{bmatrix}} & (4)\end{matrix}$

A precoding method for using a precoding matrix for leaving only blockdiagonal terms, that is, components of H(bold)_(i)B(bold face)_(i) andsetting non-block diagonal terms, which are the other components, to azero matrix 0(bold face) in the effective systematic transmission linematrix H(bold face)(bar)_(e) shown in Expression (4) is the BD method.In this embodiment, as explained below in detail, a precoding matrix fornot setting all of the non-block diagonal terms as the zero matrix0(bold face) and leaving a component of one terminal 2 other than thetransmission target terminal 2 of a transmission signal as aninterference component is used. Consequently, it is possible to secure adegree of freedom of beam formation by the transmission array andimprove a diversity gain in the transmission target terminal 2 comparedwith the BD method while suppressing IUI.

FIG. 2 is a diagram showing a configuration example of the base station1 in this embodiment. The base station 1 includes primary modulatingunits 11-1 to 11-m, a precoder unit 12, an ordering unit 13,transmission-waveform shaping units 14-1 to 14-T, antennas 15-1 to 15-T,and a receiver 16. A primary modulating units 11-i (i=1, . . . ,)performs primary modulation on a transmission signal transmitted to theterminal 2-i and outputs the primarily-modulated transmission signal tothe precoder unit 12. The primary modulation performed by the primarymodulating unit 11-i includes, for example, channel coding and mappingto a primary modulation symbol such as a QAM (Quadrature AmplitudeModulation) symbol. When a single carrier block transmission scheme isused, the primary modulation performed by the primary modulating unit11-i includes discrete Fourier transform processing as well. The primarymodulating units 11-1 to 11-m are signal generating units that generate,for each of the terminals 2, which are receiving apparatuses, atransmission signal transmitted to the terminal 2.

The precoder unit 12 is a precoder that performs precoding on signalstransmitted from the antennas 15-1 to 15-T, which are a plurality oftransmission antennas, such that received power in the terminals 2,which are one or more third receiving apparatuses excluding a firstreceiving apparatus, which is the terminal 2 serving as a transmissiondestination of transmission signals output from the primary modulatingunits 11-1 to 11-m, and second receiving apparatuses, which are theterminals 2 other than the first receiving apparatus, is 0, that is,equal to or smaller than a threshold. The first receiving apparatus is adesired terminal explained below. The second receiving apparatuses areIUI terminals explained below. The third receiving apparatuses areterminals excluding the desired terminal and the IUI terminals among theterminals 2-1 to 2-m. Received power in the IUI terminals, which are thesecond receiving apparatuses, is larger than the threshold.

Specifically, the precoder unit 12 performs precoding by multiplying thetransmission signals after the primary modulation output from theprimary modulating units 11-1 to 11-m with the precoding matrix in thisembodiment and outputs the transmission signals after the precodingrespectively to the transmission-waveform shaping units 14-1 to 14-Tcorresponding to the transmission signals. The ordering unit 13instructs the precoder unit 12 to perform ordering of the terminals 2 inthe precoding and power distribution to the terminals 2. That is, theordering unit 13 determines the order of the terminals 2 in theprecoding. The transmission-waveform shaping units 14-1 to 14-Trespectively perform secondary modulation, digital-analog (D/A)conversion, conversion from a baseband frequency into a radio frequency,and the like on the signals after the precoding and transmit the signalsafter the processing respectively via the antennas 15-1 to 15-T. Forexample, the secondary modulation is multicarrier modulation when amulticarrier scheme such as OFDM (Orthogonal Frequency DivisionMultiplex) is applied and is single carrier modulation when a singlecarrier scheme such as single carrier block transmission is applied.There is no limitation on a modulation scheme of the secondarymodulation. Modulation other than the OFDM and the single carrier blocktransmission can be performed. When block transmission such as the OFDMor the single carrier block transmission is applied, thetransmission-waveform shaping units 14-1 to 14-T perform, for example,inverse discrete Fourier transform and CP (Cyclic Prefix) additionprocessing before the D/A conversion. Note that the block transmissionindicates a scheme for converting a single into a block through thediscrete Fourier transform processing and the CP addition as representedby the OFDM and the single carrier block transmission. The signalprocessing in the transmission-waveform shaping units 14-1 to 14-T canbe digital processing or can be analog processing. Note that thetransmission signals input to the precoder unit 12 from the primarymodulating units 11-1 to 11-m correspond to s(bold face)(bar)(t) inExpression (3) and the output signals output to thetransmission-waveform shaping units 14-1 to 14-T from the precoder unit12 correspond to B(bold face)(bar)P(bold face)(bar)s(bold face)(bar)(t)in Expression (3).

Because the precoding by the precoder unit 12 is carried out, theantennas 15-1 to 15-T, which are the transmission antennas, are capableof forming a plurality of beams respectively directed to the terminals2.

The receiver 16 carries out reception processing on received signalsreceived from the terminals 2 through the antennas 15-1 to 15-T. Notethat, in an example explained here, the antennas 15-1 to 15-T aretransmission and reception antennas. However, reception antennas can beprovided separately from the antennas 15-1 to 15-T. However, when thebase station 1 uses an estimation result of an uplink transmission lineas downlink transmission line information in a calculation process of aprecoding matrix explained below, the antennas 15-1 to 15-T aretransmission and reception antennas and the receiver 16 performsestimation of a transmission line on the basis of received signalsreceived from the antennas 15-1 to 15-T. Any method can be used as anestimation method for a transmission line. For example, a method ofestimating a transmission line using a pilot signal, which is a knownsignal, can be used. Specifically, pilot signals, which are orthogonalamong the antennas of the terminal 2, are transmitted from the terminal2. The receiver 16 of the base station 1 can identify the antennas ofthe terminal 2 according to the orthogonal pilots and estimate atransmission line. In the calculation process of the precoding matrixexplained below, when the base station 1 uses transmission lineinformation received from the terminal 2, the receiver 16 outputs thereceived transmission line information to the precoder unit 12.

FIG. 3 is a diagram showing a configuration example of the terminal 2 inthis embodiment. The terminal 2 includes antennas 21-1 to 21-R,reception-waveform shaping units 22-1 to 22-R, a decoder unit 23, ademodulating unit 24, and a transmitter 25. The reception-waveformshaping units 22-1 to 22-R respectively perform processing forconverting a radio frequency into a baseband frequency, analog-digital(A/D) conversion, signal filter processing, and the like on receivedsignals received by the antennas 21-1 to 21-R and output the receivedsignals after the processing to the decoder unit 23. The signal filterprocessing is processing for extracting, for example, a signal in adesired frequency band. When a block transmission scheme is applied, thereception-waveform shaping units 22-1 to 22-R carry out CP removalprocessing and the discrete Fourier transform processing as well. Thedecoder unit 23 performs MIMO decode processing, which is processing forextracting a desired signal, that is, a signal to the own terminal, onthe received signals input from the reception-waveform shaping units22-1 to 22-R and outputs the signals after the processing to thedemodulating unit 24. The decoder unit 23 is a decoder that extracts adesired signal from signals received from the base station 1. Thedecoder unit 23 carries out estimation processing for a transmissionline in a process of the MIMO decode processing. The demodulating unit24 performs demapping processing, channel decoding processing, and thelike on the signals output from the decoder unit 23 and restores thesignals transmitted from the base station 1. When the single carrierblock transmission scheme is applied, the demodulating unit 24 carriesout equalization processing for compensating for frequency distortionand the inverse discrete Fourier transform processing. The signalprocessing in the reception-waveform shaping units 22-1 to 22-R can bedigital processing or can be analog processing.

The transmitter 25 generates a transmission signal and transmits thetransmission signal from the antennas 21-1 to 21-R to the base station1. Note that, in an example explained here, the antennas 21-1 to 21-Rare transmission and reception antennas. However, transmission antennascan be provided separately from the antennas 21-1 to 21-R. However, inthe calculation process of the precoding matrix explained below, whenthe base station 1 uses transmission line information received from theterminal 2, the transmitter 25 acquires transmission line information,which is information concerning a transmission line estimated by thedecoder unit 23, from the decoder unit 23 and transmits the transmissionline information to the base station 1. In the calculation process ofthe precoding matrix explained below, when the base station 1 uses anestimation result of an uplink transmission line as downlinktransmission line information, the antennas 21-1 to 21-R aretransmission and reception antennas. The transmitter 25 transmitstransmission signals from the antennas 21-1 to 21-R.

Hardware configurations of the base station 1 and the terminal 2 in thisembodiment are explained. The components configuring the base station 1shown in FIG. 1 can be respectively realized as hardware such aselectronic circuits and antennas. The primary modulating units 11-1 to11-m are mappers or modulators. When the primary modulation includes thediscrete Fourier transform processing, a discrete Fourier transformprocessing circuit is added. The precoder unit 12 is a processingcircuit that carries out precoding. The ordering unit 13 is a processingcircuit that performs ordering. The transmission-waveform shaping units14-1 to 14-T are transmission-waveform shaping circuits and,specifically, configured by D/A converters, frequency converters, andthe like. When the transmission-waveform shaping units 14-1 to 14-Tperform the CP addition and the inverse discrete Fourier transformprocessing, the transmission-waveform shaping units 14-1 to 14-T includeCP addition circuits and inverse discrete Fourier transform processingcircuits.

The processing circuit that realizes the precoder unit 12 and theordering unit 13 can be dedicated hardware or can be a control circuitincluding a memory and a CPU (Central Processing Unit; also referred toas central processing device, processing device, arithmetic device,microprocessor, microcomputer, processor, or DSP (Digital SignalProcessor)) that execute programs stored in the memory. For example, anonvolatile or volatile semiconductor memory such as a RAM (RandomAccess Memory), a ROM (Read Only Memory), a flash memory, an EPROM(Erasable Programmable Read Only Memory), or an EEPROM (ElectricallyErasable Programmable Read Only Memory), a magnetic disk, a flexibledisk, an optical disk, a compact disk, a minidisk, or a DVD (DigitalVersatile Disk) corresponds to the memory.

When the precoder unit 12 and the ordering unit 13 are realized bydedicated hardware, the precoder unit 12 and the ordering unit 13 are,for example, single circuits, composite circuits, programmed processors,parallel-programmed processors, ASICs (Application Specific IntegratedCircuits), FPGAs (Field Programmable Gate Arrays), or combinations ofthe foregoing. When the processing circuit is realized by dedicatedhardware, the processing circuit is, for example, a processing circuit500 shown in FIG. 4.

when the precoder unit 12 and the ordering unit 13 are realized by acontrol circuit including a CPU, the control circuits is, for example, acontrol circuit 400 having a configuration shown in FIG. 5. As shown inFIG. 5, the control circuit 400 includes a processor 401, which is aCPU, and a memory 402. When the precoder unit 12 and the ordering unit13 are realized by the control circuit 400 as shown in FIG. 5), theprecoder unit 12 and the ordering unit 13 are realized by the processor401 reading out and executing programs corresponding to respective kindsof processing by the precoder unit 12 and the ordering unit 13 stored inthe memory 402. The memory 402 is also used as a temporary memory inprocessing carried out by the processor 401.

At least a part of the primary modulating units 11-1 to 11-m and thetransmission-waveform shaping units 14-1 to 14-T can be realized by theprocessing circuit, which is the dedicated hardware, or the controlcircuit 400 like the precoder unit 12 and the ordering unit 13.

The components configuring the terminal 2 shown in FIG. 2 can berespectively realized as hardware such as electronic circuits andantennas. The reception-waveform shaping units 22-1 to 22-R arereception-waveform shaping circuits and, specifically, configured by A/Dconverters, filters, frequency converters, and the like. When thereception-waveform shaping units 22-1 to 22-R perform the CP removal andthe discrete Fourier transform processing, the reception-waveformshaping units 22-1 to 22-R include CP removal circuits and discreteFourier transform processing circuits. The decoder unit 23 is aprocessing circuit. The demodulating unit 24 is a demodulator or ademapper. When the demodulating unit 24 performs the equalizationprocessing, the inverse discrete Fourier transform processing, and thelike, the demodulating unit 24 includes an equalizer, an inversediscrete Fourier transform circuit, and the like.

The processing circuit that realizes the decoder unit 23 can be realizedby dedicated hardware or can be realized by the control circuit 400.When the decoder unit 23 is realized by the control circuit 400 as shownin FIG. 5, the decoder unit 23 is realized by the processor 401 readingout and executing a program corresponding to the processing by thedecoder unit 23 stored in the memory 402. At least a part of thereception-waveform shaping units 22-1 to 22-R and the demodulating unit24 can be realized by the processing circuit, which is the dedicatedhardware, or the control circuit 400 like the decoder unit 23.

The precoding processing carried out by the precoder unit 12 in thisembodiment is explained. The system model in the communication system inthis embodiment is as explained with reference to Expression (1) andExpression (2). The precoder unit 12 generates a precoding matrixaccording to a procedure explained below. Note that, in the followingexplanation, processing by the transmission-waveform shaping units 14-1to 14-T and the reception-waveform shaping units 22-1 to 22-R is omittedfor explanation in formulas. However, there is no influence of theprocessing in calculation of a precoding matrix. In the followingexplanation, a space between an output end of the precoder unit 12 ofthe base station 1 and an input end of the decoder unit 23 of theterminal is represented by an equivalent low-pass system. The precodingprocessing explained below can be independently carried out at eachdiscrete frequency in the OFDM or the single carrier block transmissionor can be collectively carried out in an entire band irrespective of afrequency.

In a process of the precoding matrix calculation explained below,information concerning a transmission line matrix in a downlinkdirection, that is, transmission line information is necessary. A methodin which the precoder unit 12 acquires a transmission line matrix is notparticularly limited. However, for example, when the communicationsystem is a communication system that adopts frequency division duplex(FDD) in which communication is performed at different frequencies in adownlink and an uplink, transmission line information estimated in theterminal 2 and received from the terminal 2 is used. When thecommunication system is a communication system in which a downlink andan uplink are performed by time division duplex (TDD), reversibility oftransmission and reception can be used. Therefore, in this case, thereceiver 16 can estimate a transmission line in an uplink direction onthe basis of a signal received from the terminal 2 and use the estimatedtransmission line as transmission line information of the downlink. As amethod for transmission line estimation, any method can be used asexplained above. For example, a method of estimating a transmission lineusing a pilot signal can be used.

FIG. 6 is a flowchart showing an example of a processing procedure inthe precoder unit 12 in this embodiment. In the following explanation, aterminal set as the terminal 2 at a destination of a transmission signalis referred to as desired terminal. First, the precoder unit 12determines the desired terminal according to order determined by theordering unit 13. To calculate a precoding matrix for the desiredterminal, the precoder unit 12 selects IUI terminals, which are theterminals 2 that allow IUI, and corresponding to the desired terminal(step S1).

As explained above, in the BD method, for terminals other than thedesired terminal, a beam is formed to form null to avoid IUI. On theother hand, in this embodiment, IUI is allowed for one terminal 2 otherthan the desired terminal. That is, a beam is formed not to form nullfor one terminal 2 other than the desired terminal and to havedirectivity in the directions of one terminal 2 other than the desiredterminal.

As a selection method for an IUI terminal, when the desired terminal isrepresented as a terminal 2-i, there are, for example, a method ofselecting a terminal including a transmission line matrix having a lowcorrelation with H(bold face)_(i), which is a transmission line matrixof the terminal 2-i, and a method of selecting the terminal 2 in aposition away from the desired terminal on the basis of geographicalinformation of the terminals 2. The former is a method of selecting anIUI terminal corresponding to the desired terminal on the basis of acorrelation between a transmission line matrix between the desiredterminal and the base station 1 and a transmission line matrix betweenthe terminal 2 other than the desired terminal and the base station 1.For example, when the terminal 2 other than the desired terminal isrepresented as a terminal 2-k, the precoder unit 12 calculates,concerning all the terminals 2 other than k=i, a square sum of diagonalterms of a cross-correlation matrix H(bold face)_(k) ^(H)H(boldface)_(i) between the transmission line matrix H(bold face)_(i) and atransmission line matrix H(bold face)_(k) of the terminal 2-k andselects the terminal 2 having the smallest square sum of the diagonalterms of H(bold face)_(k) ^(H)H(bold face)_(i). The latter is a methodof selecting the terminal 2 on the basis of geographical separationdegrees between the desired terminal and the terminals 2 other than thedesired terminal. When the latter selection method is used, for example,the base station 1 calculates, on the basis of position information ofthe terminals 2 and position information of the base station 1, for eachof the terminals 2, an azimuth angle of the terminal 2 estimated fromthe base station 1 and selects the terminal 2 having an azimuth anglemost away from the desired terminal. The base station 1 acquires theposition information of the terminal 2 by, for example, receiving, fromthe terminal 2, position information calculated by the terminals 2 usinga GPS (Global Positioning System). As the position information of thebase station 1, for example, the position information calculated usingthe GPS is used. However, a user selected as the IUI terminal once isnot selected as the IUI terminal during calculation of a precodingmatrix when another terminal 2 is set as the desired terminal. That is,the same terminal 2 is not redundantly selected as the IUI terminal.

Note that, as explained below, for example, when ordering is carried outby the ordering unit 13 such that a square sum of diagonal terms of thecross-correlation matrix H(bold face)_(k) ^(H)H(bold face)_(i) betweenthe terminal 2 and the adjacent, that is, following terminal 2decreases, the terminal 2 having the next index of the desired terminal2 can be selected as the IUI terminal.

Subsequently, when the IUI terminal is represented as terminal 2-j, theprecoder unit 12 calculates a matrix H (bold face)(bar)_(i,j) obtainedby excluding transmission line components of the desired terminal andthe IUI terminal from a system transmission line matrix (step S2) andperforms singular value decomposition (SVD) of H(bold face)(bar)_(i,j)(step S3).

As explained above, a system transmission line matrix H(bold face) is amatrix indicating transmission lines from the antennas of the basestation 1 to all the branches of all the terminals 2 after themultiplication of the reception weight and can be calculated on thebasis of a transmission line matrix of each of the terminals 2. A matrixH(bold face)(bar)_(i,j), which is a (N_(w, total)−2N_(w))×T matrixobtained by excluding the transmission line components of the desiredterminal and the IUI terminals from the system transmission line matrixH(bold face), can be represented by the following Expression (5). Asindicated by Expression (5), the singular value decomposition of theH(bold face)(bar)_(i,j) can be performed.

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 5} \right\rbrack & \; \\{{{\overset{\_}{H}}_{i,j}\begin{bmatrix}\vdots \\H_{i - 1} \\H_{i + 1} \\\vdots \\H_{j - 1} \\H_{j + 1} \\\vdots\end{bmatrix}} = {{U_{i,j}{\sum_{i,j}V_{I,j}^{H}}} = {{U_{i,j}\begin{bmatrix}\sum_{i,j}^{(s)} & O\end{bmatrix}}\begin{bmatrix}V_{i,j}^{{(s)}H} \\V_{i,j}^{{(n)}H}\end{bmatrix}}}} & (5)\end{matrix}$

In the expression, U(bold face)_(i,j) is a left singular vector matrixof H(bold face)(bar)_(i,j), V(bold face)_(i,j) is a right singularvector matrix, and Σ(bold face)_(i,j) is a singular value matrix havingsingular values in diagonal terms. In Σ(bold face)_(i,j), when thesingular values of the diagonal terms are in descending order of powersaccording to magnitudes, as indicated by Expression (5), the singularvalues can be represented by being divided into a partial diagonalmatrix Σ(bold face)_(i) ^((s)) configured by (N_(w, total)−2N_(w))non-zero singular values and a zero matrix 0(bold face) corresponding to(T−(N_(w, total)−2N_(w))) zero singular values. Right singular vectorsV(bold face)_(i,j) ^((s)) and V(bold face)_(i,j) ^((n)) respectivelycorresponding to Σ(bold face)_(i,j)(s) and the zero matrix 0(bold face)are present. When V(bold face)_(i,j) ^((n)), which is a first matrix, isa precoding matrix of the terminal 2-i, an effective transmission linematrix for the terminal 2-i can be represented by the followingExpression (6). When the precoding matrix is used, null-steering isperformed for the terminals 2 other than the terminal 2-i and theterminal 2-j. When the precoding is carried out using the first matrix,null is formed for the terminals 2 excluding the terminal 2-i and theterminal 2-j. The null indicates that received power of signalstransmitted from the antennas 15-1 to 15-T is equal to or smaller than athreshold, for example, the received power is 0.

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 6} \right\rbrack & \; \\{{\overset{\_}{H}}_{e,i} = {{\overset{\_}{H}V_{i,j}^{(n)}} = \begin{bmatrix}\vdots \\O \\{H_{i}V_{i,j}^{(n)}} \\O \\\vdots \\{H_{j}V_{ij}^{(n)}} \\O \\\vdots\end{bmatrix}}} & (6)\end{matrix}$

Referring back to FIG. 6, the precoder unit 12 calculates an eigenvectormatrix V(bold face)_(i,j) ^((e)), which is a second matrix, from adesired component H(bold face)_(i)V(bold face)_(i,j) ^((n)), which is acomponent corresponding to the terminal 2-i in Expression (6) (step S4).That is, the precoder unit 12 generates the second matrix for forming abeam space suitable for the terminal 2-i, that is, directed to theterminal 2-i. Specifically, that is, the precoder unit 12 performs thesingular value decomposition of H(bold face)_(i)V(bold face)_(i,j)^((n)) or applies the singular value decomposition to a non-negativevalue Hermitean matrix (H(bold face)_(i)V(bold face)_(i,j)^((n)))^(H)H(bold face)_(i)V(bold face)_(i,j) ^((n)) of H(boldface)_(i)V(bold face)_(i,j) ^((n)) and calculates an eigenvector matrixV(bold face)_(i,j) ^((e)) corresponding to a large eigenvalue. The largeeigenvalue is an eigenvalue on a forward side at the time when aplurality of eigenvalues are arranged in descending order of powers.

Subsequently, the precoder unit 12 calculates a precoding matrixcorresponding to the desired terminal, that is, the terminal 2-i (stepS5). Specifically, the precoder unit 12 calculates the precoding matrixcorresponding to the terminal 2-i according to the following Expression(7). In this embodiment, as indicated by Expression (7), it is possibleto realize beam formation for, by multiplying a transmission signal withV(bold face)_(i,j) ^((n)), after performing null-steering on a spaceexcluding the terminal 2-i and the terminal 2-j, multiplying the spacewith V(bold face)_(i,j)(e) to form signal spaces for the terminal 2-iand the terminal 2-j and then improve a reception gain in the terminal2-i. That is, in this embodiment, signals transmitted from thetransmission antennas are multiplied with the first matrix, which is theprecoding matrix for reducing received power in the terminals 2excluding the desired terminal and the IUI terminals to the threshold orless. A multiplication result is multiplied with the second matrix,which is the precoding matrix for forming a beam directed to the desiredterminal.

[Math. 7]

B _(i) =V _(i,j) ^((n)) V _(i,j) ^((e))  (7)

The precoder unit 12 determines whether the processing, that is, thecalculation processing of the precoding matrix is ended for all theterminals 2 (step S6). When the processing is ended for all theterminals 2 (Yes at step S6), the precoder unit 12 calculates a systemprecoding matrix B(bold face)(bar) (step S7) and ends the processing.The system precoding matrix B (bold face)(bar) is a matrix in whichprecoding matrixes for each of the terminals 2 are arranged in a columndirection. When the processing is not ended for all the terminals 2 (Noat step S6), the precoder unit 12 changes the desired terminal andreturns to step S1. In step S1, the precoder unit 12 selects IUIterminals corresponding to the changed desired terminals, and thenperforms processes of step 2 and subsequent steps.

First, the precoder unit 12 multiplies transmission signals output fromthe primary modulating units 11-1 to 11-m with a power distributionmatrix P(bold face)(bar) generated on the basis of power distributionnotified from the ordering unit 13, multiplies a multiplication resultwith the system precoding matrix B(bold face)(bar) calculated by theprocessing explained above, and outputs a multiplication result to thetransmission-waveform shaping units 14-1 to 14-T. That is, the precoderunit 12 multiplies signals transmitted from the transmission antennaswith a power distribution matrix corresponding to a result of the powerdistribution and the system precoding matrix B(bold face)(bar), which isa precoding matrix for carrying out the precoding. As it is seen fromExpressions (1), (2), and (3), the power distribution matrix is a matrixhaving, as a diagonal element, a square root of electric power P(boldface), distributed to the terminal 2-i. The transmission-waveformshaping units 14-1 to 14-T perform the processing explained above andtransmit the signals after the processing from the antennas 15-1 to15-T.

For example, m=4. That is, when four terminals 2 are present, it isassumed that the precoder unit 12 selects, as IUI terminals of thedesired terminal, the terminal 2 having the next index of the desiredterminal. That is, concerning beam formation to the terminal 2-1,interference to the terminal 2-2 is allowed, concerning beam formationto the terminal 2-2, interference to the terminal 2-3 is allowed,concerning beam formation to the terminal 2-3, interference to theterminal 2-4 is allowed, and, concerning beam formation to the terminal2-4, interference to the terminal 2-1 is allowed. In this case, aneffective system transmission line matrix to which the system precodingmatrix B(bold face)(bar) in this embodiment is applied can berepresented by the following Expression (8).

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 8} \right\rbrack & \; \\{{\overset{\_}{H}}_{e} = \begin{bmatrix}{H_{1}B_{1}} & O & O & {H_{1}B_{4}} \\{H_{2}B_{1}} & {H_{2}B_{2}} & O & O \\O & {H_{3}B_{2}} & {H_{3}B_{3}} & O \\O & O & {H_{4}B_{3}} & {H_{4}B_{4}}\end{bmatrix}} & (8)\end{matrix}$

Concerning an average received signal power-to-noise power ratio(Signal-to-Noise power Ratio: SNR) in the desired terminal, animprovement effect by the precoding is quantitatively indicated from asimulation result. FIG. 7 shows an average SNR during precodingapplication with respect to an average SNR during precodingnon-application in the case of the number of base station transmissionantennas T=16, the number of terminals m=8, and the number of terminalreception branches N_(w)=2. Therefore, a system transmission line matrixis a matrix of 16×16. Elements in the system transmission line matrixare independent and similar complex Gaussian random numbers. The numberof times of random number trials is set to 10000 times. A broken line inthe figure indicates a characteristic (conventional) at the time whenthe precoding by the conventional BD method is applied. A solid lineindicates a characteristic (invented) at the time when the precoding inthis embodiment is applied. It is seen from FIG. 7 that the precoding inthe embodiment of the present invention can improve an average SNR morethan the conventional BD method. For example, when compared in a case inwhich an SNR during precoding non-application is 20 decibels, theaverage SNR can be improved by 6.5 decibels with respect to theconventional BD method by applying the precoding in the embodiment ofthe present invention. This is because a diversity effect is obtained byforming a beam directed to the desired terminal as explained above.

Processing by the ordering unit 13 is explained. To enable the precoderunit 12 to easily select the IUI terminal, arrangement order of theterminals 2 is important. The ordering unit 13 determines thearrangement order of the terminal 2. The ordering unit 13 determinespower distribution to the terminals 2.

FIG. 8 is a flowchart showing an example of a processing procedure ofthe ordering unit 13 in this embodiment. The ordering unit 13 determinesthe order of the terminals 2 (step S11). The ordering unit 13 notifiesthe determined order to the precoder unit 12. Examples of a method ofordering include a method of ordering the terminals 2 in descendingorder or ascending order of transmission line gains (squares of aFrobenius norm of a transmission line matrix) of the terminals 2, amethod of ordering the terminals 2 in descending order or ascendingorder of non-negative eigenvalues or non-negative singular values oftransmission line matrixes between the base station 1 and the terminals2, a method of ordering the terminals 2 such that geographical positionsof adjacent terminals, for example, azimuth angles viewed from the basestation 1 are close to each other or different from each other, and amethod of ordering the terminals 2 such that a correlation betweentransmission line matrixes of the adjacent terminals, that is, themagnitude of diagonal terms of a cross correlation matrix of atransmission line matrix between the terminals increases or decreases.However, the method of ordering is not limited to these methods.

The order of the ordering by the ordering unit 13 can be set to orderdecided such that the terminal 2 in the next order of the desiredterminal is the IUI terminal corresponding to the desired terminal.Examples of such ordering include a method of ordering the terminals 2such that geographical positions of the adjacent terminals are close toeach other or distant from each other, that is, ordering the terminals 2such that the terminals continuous in order are geometrically close toeach other or separated from each other and a method of ordering theterminals 2 such that a correlation between transmission line matrixesof the adjacent terminals increases or decreases, that is, ordering theterminals 2 such that a correlation between transmission line matrixesof the terminals 2 continuous in order increases or decreases. When theterminals 2 are ordered such that the azimuth angles viewed from thebase station 1 are different from each other, for example, the terminals2 corresponding to indexes are selected in such a manner as to selectany terminal 2 as a first terminal 2, select, as a second terminal 2, aterminal that is in a most distant geographical position from the firstterminal 2, and select, as a third terminal 2, the terminal 2 that is ina most distant geographical position from the second terminal 2 and isnot ordered.

The ordering unit 13 determines power distribution to the terminals 2(step S12). The ordering unit 13 notifies a result of the powerdistribution, that is, electric power distributed to the terminals 2 tothe precoder unit 12. For example, the precoder unit 12 carries out thepower distribution according to a water filling principle on the basisof a transmission line gain of the terminals 2. Alternatively, theprecoder unit 12 carries out the power distribution to equalizereception quality of all the terminals 2, that is, such that a productof the transmission line gain and the distributed power is an equivalentvalue among all the terminals 2. However, the power distribution is notlimited to the above. Note that the order of step S11 and step S12 canbe opposite.

Processing in the decoder unit 23 of the terminal 2 is explained. Atransmission line component observed by the terminal 2, which is thereceiving apparatus that receives a beam formed by the base station 1using the system precoding matrix in this embodiment explained above, isconsidered. It is assumed that, in the terminal 2-i, a signaltransmitted to the terminal 2-k by the base station 1 is observed as aninterference signal. That is, it is assumed that, in the base station 1,the terminal 2-i is selected as an IUI terminal Of the terminal 2-k. Atthis point, a received signal received by the terminal 2-i includes adesired transmission line component H(bold face)_(i)B(bold face)_(i) andan interfered component H(bold face)_(i)B(bold face)_(k) by the signaltransmitted to the terminal 2-k. Therefore, a received signal r(boldface)_(i)(t) received by the terminal 2-i can be represented by thefollowing Expression (9). In the expression, s(bold face)_(i)(t) is atransmission signal transmitted from the base station 1 to the terminal2-i and s(bold face)_(k)(t) is a transmission signal transmitted fromthe base station 1 to the terminal 2-k.

[Math. 9]

r _(i)(t)=H _(i) B _(i)√{square root over (P _(i))}s _(i)(t)+H _(i) B_(k)√{square root over (P _(k))}s _(k)(t)+n _(i)(t)  (9)

The decoder unit 23 of the terminal 2 detects, from the received signalr(bold face)_(i)(t), the transmission signal s(bold face)_(i)(t)transmitted to the terminal 2-i. The detection of the transmissionsignal s(bold face)_(i)(t) from the received signal r(bold face)_(i)(t)can be realized by general MIMO decode processing. For example, asdescribed in “T. Ohgane, T. Nishimura, and Y. Ogawa, “Applications ofSpace Division Multiplexing and Those Performance in a MIMO Channel,“IEICE Trans. Commun., vol. E88-B, no. 5, pp. 1843-1851, May 2005.”, alinear detection method represented by ZF (Zero-Forcing) and a minimummean square error (MMSE) standard can be applied. Alternatively, anonlinear detection method represented by maximum likelihood estimationor an interference canceller (IC) can also be applied. Any MIMO decodeprocessing can be used. Note that the MIMO decode processing performedby the decoder unit 23 can be carried out on y(bold face)_(i)(t) beforereception weight multiplication instead of being carried out on r(boldface)_(i)(t) after the reception weight multiplication. MIMO decodeprocessing in this case is also the same as the general MIMO decodeprocessing.

In the above explanation, the number of antennas T of the base station 1and the number of reception branches N_(w) of the terminal 2 satisfy therelation of T≧N_(w,total)−N_(w)=(m−1)×N_(w). However, there is nolimitation on the number of antennas included in the terminal 2. Thepresent invention is also applicable when the number of antennas isdifferent for each of the terminals 2 and when the number of receptionbranches is different for each of the terminals 2. When the number ofantennas N_(R,j) and the number of branches N_(w,j) of the terminal 2-jsatisfy a relation of N_(R,j)≧N_(w,j) and the IUI terminal for theterminal 2-i, which is the desired terminal, is the terminal 2-j, thepresent invention is applicable if T≧(Σ_(k-1) ^(m)(N_(w,k)))−N_(w,j) issatisfied in a relation between the base station 1 and all desiredterminals.

In FIG. 2 referred to above, the example is shown in which the basestation 1 includes the ordering unit 13. However, the base station 1 caninclude a configuration shown in FIG. 9 without including the orderingunit 13. FIG. 9 is a diagram showing a configuration example of a basestation 1 a not including the ordering unit 13. In FIG. 9, componentshaving the same functions as the functions of the base station 1 shownin FIG. 2 are denoted by the same reference numerals and signs as thosein the base station 1 shown in FIG. 2. In the base station 1 a shown inFIG. 9, rearrangement by the ordering unit 13 is not performed. However,the precoder unit 12 can select IUI terminals according to the selectionmethod explained above and carry out the operation explained above.Consequently, in the base station 1 a shown in FIG. 9, it is possible toform a beam that forms null for the terminals 2 other than the desiredterminal and the IUI terminals as in the beam formation by the basestation 1 shown in FIG. 2.

As explained above, in this embodiment, the base station 1 decides, foreach of the terminals 2, the IUI terminals that allow interference andforms a beam that forms null for the terminals 2 other than the desiredterminal and the IUI terminals. Therefore, it is possible to improve adiversity gain in the transmission target terminal 2 compared with theBD method while suppressing IUI.

Second Embodiment

FIG. 10 is a diagram showing the configuration of a control station 3and base stations 1 b-1 to 1 b-q controlled by the control station 3 ina second embodiment according to the present invention. In the figure, qis an integer equal to or larger than two. In the first embodiment, theexample is explained in which beams are formed by antennas 15-1 to 15-Tmounted on the base station 1. However, not only this, but, when Tantennas are distributedly mounted on a plurality of base stations, thesame system precoding matrix as the system precoding matrix in the firstembodiment can be used. When the base stations 1 b-1 to 1 b-q areexplained without being distinguished, the base stations 1 b-1 to 1 b-qare described as base stations 1 b. In this embodiment, a total numbersof the numbers of antennas included in the base stations 1 b-1 to 1 b-qis T.

The control station 3 shown in FIG. 10 includes a precoder calculationunit 31, an ordering unit 32, and a transceiver 33. The precodercalculation unit 31 carries out the same processing as the processing bythe precoder unit 12 in the first embodiment. That is, the precodercalculation unit 31 calculates a precoding matrix for performingprecoding such that received power in the terminals 2 excluding desiredterminals, which are the terminals 2 at transmission destinations oftransmission signals transmitted by the base stations 1 b-1 to 1 b-q,and IUI terminals, which are the terminals 2 other than the desiredterminals, is equal to or smaller than a threshold. However, theprecoder calculation unit 31 receives transmission line information usedfor calculation of a system precoding matrix from the base stations 1b-1 to 1 b-q through the transceiver 33. A method in which the basestations 1 b-1 to 1 b-q acquire the transmission line information is thesame as the method in the first embodiment. The ordering unit 32 carriesout the same processing as the processing by the ordering unit 13 in thefirst embodiment. The transceiver 33 performs reception processing forsignals received from the base stations 1 b-1 to 1 b-q and transmissionprocessing for signals transmitted to the base stations 1 b-1 to 1 b-q.The transceiver 33 transmits the system precoding matrix, which is theprecoding matrix calculated by the precoder calculation unit 31, andpower distribution calculated by the ordering unit 32 respectively tothe base stations 1 b-1 to 1 b-q. Each of the base stations 1 b-1 to 1b-q includes one or more transmission antennas.

FIG. 11 is a diagram showing a configuration example of the base station1 b in this embodiment. As shown in FIG. 11, the base station 1 b is thesame as the base station 1 in the first embodiment except that atransceiver 17 is added to the base station 1 in the first embodimentand the base station 1 b includes a precoder unit 12 a instead of theprecoder unit 12. However, the number of the transmission-waveformshaping units and the number of antennas are respectively c; c is aninteger equal to or larger than one. Components having the samefunctions as the functions in the first embodiment are denoted by thesame reference numerals and signs as those in the first embodiment andredundant explanation is omitted.

The transceiver 17 performs reception processing for a signal receivedfrom the control station 3 and transmission processing for a signal tobe transmitted to the control station 3. The transceiver 17 acquirestransmission line information from the receiver 16 and transmits thetransmission line information to the control station 3. The transceiver17 outputs a system precoding matrix and power distribution receivedfrom the control station 3 to the precoder unit 12 a. The precoder unit12 a multiplies transmission signals output from the primary modulatingunits 11-1 to 11-m with a power distribution matrix P(bold face)_(i)generated on the basis of the power distribution received from thetransceiver 17, further multiplies the transmission signals with thesystem precoding matrix B(bold face)(bar) received from the transceiver,17 and outputs a multiplication result to transmission-waveform shapingunits 14-1 to 14-c.

Hardware configurations of the control station 3 and the base station 1b are explained. The same components as the components in the firstembodiment among components of the base station 1 b can be realized bythe hardware configuration explained in the first embodiment. Theprecoder calculation unit 31 and the ordering unit 32 of the controlstation 3 are processing circuits. Like the processing circuit thatrealizes the precoder unit 12 and the ordering unit 13 in the firstembodiment, the precoder calculation unit 31 and the ordering unit 32can be dedicated hardware or can be a control circuit including a memoryand a CPU that executes a program stored in the memory. The controlcircuit that realizes the precoder calculation unit 31 and the orderingunit 32 is, for example, the control circuit 400 shown in FIG. 5. Theprecoder unit 12 a is also a processing circuit. The processing circuitcan be dedicated hardware or can be a control circuit including a memoryand a CPU that executes a program stored in the memory. The controlcircuit that realizes the precoder unit 12 a is, for example, thecontrol circuit 400 shown in FIG. 5.

The transceiver 33 of the control station 3 is configured by atransmitter and a receiver. The transceiver 17 of the base station 1 bis also configured by a transmitter and a receiver.

As explained above, in this embodiment, the control station 3 calculatesthe same system precoding matrix B(bold face)(bar) as the systemprecoding matrix in the first embodiment and notifies the systemprecoding matrix B(bold face)(bar) to the base station 1 b. Therefore,even when the communication system includes a plurality of base stations1 b, it is possible to obtain the same effects as the effects in thefirst embodiment.

Third Embodiment

FIG. 12 is a diagram showing a configuration example of a base station 1c in a third embodiment according to the present invention. The basestation 1 c in this embodiment is the same as the base station 1 in thefirst embodiment except that the precoder unit 12 of the base station 1in the first embodiment is replaced with a precoder unit 12 b and anonlinear processing unit 18 is added. The terminals 2-1 to 2-m in thisembodiment are the same as the terminals 2-1 to 2-m in the firstembodiment. Components having the same functions as the functions in thefirst embodiment are denoted by the same reference numerals and signs asthose in the first embodiment and redundant explanation is omitted.

In this embodiment, in the terminal 2-1 to the terminal 2-(m-1), as inthe first embodiment, one IUI terminal is selected for each of desiredterminals and a precoding matrix is calculated. Note that it is assumedthat branch numbers of the signs of the terminal 2-1 to the terminal2-(m-1) indicate order after ordering by the ordering unit 13 and theordering unit 13 performs rearrangement such that the terminal 2 havingthe next index of the desired terminal can be selected as the IUIterminal. For example, as explained in the first embodiment, it isassumed that the terminals 2 are ordered such that geographicalpositions of terminals having continuous indexes are away or acorrelation of transmission line matrixes decreases. In this embodiment,concerning the terminal 2-m, which is the last terminal 2, a precodingmatrix that does not allow interference and realizes perfect nullsteering for the other terminals is calculated.

FIG. 13 is a flowchart showing an example of a processing procedure inthe precoder unit 12 b in this embodiment. The precoder unit 12 bselects a desired terminal as in the first embodiment and determineswhether the desired terminal is an m-th terminal 2 (step S20). When thedesired terminal is not the m-th terminal 2 (No at step S20), theprecoder unit 12 b proceeds to step S1. Step S1 to step S7 are the sameas the steps in the first embodiment.

When the desired terminal is the m-th terminal 2 (Yes at step S20), theprecoder unit 12 b calculates, concerning the terminal 2-m, a matrixH(bold face)_(m) obtained by removing a transmission line component ofthe desired terminal from the system transmission line matrix indicatedby Expression (9) (step S21). As indicated by Expression (10), thefollowing singular value decomposition of the matrix H(boldface)(bar)_(m) can be performed. The precoder unit 12 b performs thesingular value decomposition as indicated by Expression (10) (step S22).

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 10} \right\rbrack & \; \\{{\overset{\_}{H}}_{m} = {\begin{bmatrix}H_{1} \\H_{2} \\\vdots \\H_{m - 1}\end{bmatrix} = {{U_{m}{\sum_{m}V_{m}^{H}}} = {{U_{m}\begin{bmatrix}\sum_{m}^{(s)} & O\end{bmatrix}}\begin{bmatrix}V_{m}^{{(s)}H} \\V_{m}^{{(n)}H}\end{bmatrix}}}}} & (10)\end{matrix}$

In the expression, U(bold face)_(m′) is a left singular vector matrix ofH(bold face)(bar)_(m′), V(bold face)_(m′) is a right singular vectormatrix, and Σ(bold face)_(m′) is a singular value matrix having singularvalues in diagonal terms. In Σ(bold face)_(m), when the singular valuesof the diagonal terms are in descending order of powers according tomagnitudes, as indicated by Expression (10), the singular values can berepresented by being divided into a partial diagonal matrix Σ(boldface)_(m′) ^((s)) configured by (N_(w, total)−N_(w)) non-zero singularvalues and a zero matrix 0(bold face) corresponding to(T−(N_(w, total)−N_(w))) zero singular values. Right singular vectorsV(bold face)_(m′) ^((s)) and V(bold face)_(m′) ^((n)) respectivelycorresponding to Σ(bold face)_(m)(s) and the zero matrix 0(bold face)are present. When V(bold face)_(m′) ^((n)) is a precoding matrix of theterminal 2-m, an effective transmission line matrix for the terminal 2-mcan be represented by the following Expression (11). When the precodingmatrix is used, null-steering is performed for the terminals 2 otherthan the terminal 2-m. That is, for terminals other than the terminal2-m, null is formed.

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 11} \right\rbrack & \; \\{{\overset{\_}{H}}_{e,m} = {{\overset{\_}{H}V_{m}^{(n)}} = {\begin{bmatrix}{H_{1}V_{m}^{(n)}} \\{H_{2}V_{m}^{(n)}} \\\vdots \\{H_{m - 1}V_{m}^{(n)}} \\{H_{m}V_{m}^{(n)}}\end{bmatrix} = \begin{bmatrix}O \\O \\\vdots \\O \\{H_{m}V_{m}^{(n)}}\end{bmatrix}}}} & (11)\end{matrix}$

Referring back to FIG. 13, the precoder unit 12 b calculates concerningV(bold face)_(m) ^((n)) as a precoding matrix of the terminal 2-m on thebasis of Expression (10) (step S23) and proceeds to step S6. In thisembodiment, at step S7, the precoding unit 12 b generates a systemprecoding matrix using the precoding matrix calculated at step S5 as aprecoding matrix of the terminals 2-1 to 2-(m-1) and using the precodingmatrix calculated at step S23 as a precoding matrix of the terminal 2-m.That is, a precoding matrix for setting the m-th terminal 2, which isthe last terminal among the terminals ordered by the ordering unit 13,as the desired terminal is a matrix for performing the precoding suchthat received power in fourth receiving apparatuses, which are theterminals 2-1 to 2-(m-1) excluding the desired terminal, is equal to orsmaller than the threshold. On the other hand, a precoding matrix forsetting the terminal 2 other than the m-th terminal 2 as the desiredterminal is a matrix for performing the precoding such that receivedpower in third receiving apparatuses, which are the terminals 2-1 to 2-mexcluding the desired terminal and the IUI terminal, is equal to orsmaller than the threshold.

When the precoding matrix explained above is applied to a systemtransmission line, an effective system transmission line matrixindicated by Expression (12) is observed.

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 12} \right\rbrack & \; \\{{\overset{\_}{H}}_{e} = \begin{bmatrix}{H_{1}B_{1}} & O & O & \ldots & O & O \\{H_{2}B_{1}} & {H_{2}B_{2}} & O & \ldots & O & O \\O & {H_{3}B_{2}} & {H_{3}B_{3}} & \ddots & \vdots & \vdots \\\vdots & \vdots & \ddots & \ddots & \ddots & \vdots \\O & O & \ldots & {H_{m - 1}B_{m - 2}} & {H_{m - 1}B_{m - 1}} & O \\O & O & \ldots & O & {H_{m}B_{m - 1}} & {H_{m}B_{m}}\end{bmatrix}} & (12)\end{matrix}$

As it is seen from Expression (12), unlike Expression (8) in the firstembodiment, the effective system transmission line matrix in thisembodiment is subjected to a block double diagonalization. That is,layering is realized in which a component corresponding to a desiredterminal is present in diagonal components and components of IUIterminals are present under the diagonal components, that is, in secondlayer. Consequently, it is possible to apply nonlinear MU-MIMOprocessing for performing sequential interference removal on atransmission side as explained below.

The precoder unit 12 b outputs the system precoding matrix B(boldface)(bar) calculated by the processing explained above, transmissionsignals output from the primary modulating units 11-1 to 11-m, and powerdistribution to the nonlinear processing unit 18. The nonlinearprocessing unit 18 carries out processing for removing, on atransmission side, in advance, components to be an interference signalon a reception side as explained below using the block doublediagonalization.

The nonlinear processing unit 18 carries out the nonlinear MU-MIMOprocessing on a signal output from the precoder unit 12 b. According toExpression (12), when the signal output from the precoder unit 12 b isreceived by the terminal 2-i, the received signal can be represented bythe following Expression (13).

[Math. 13]

r _(i)(t)=H _(i) B _(i)√{square root over (P _(i))}s _(i)(t)+H _(i) B_(i-1)√{square root over (P _(i-1))}s _(i-1)(t)+n _(i)(t)  (13)

If transmission signals s(bold face)_(i-1)(t) for a terminal 2-(i-1) isknown, it is possible to remove interference on the reception side bycorrecting s(bold face)_(i)(t) to a signal given by Expression (14).

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 14} \right\rbrack & \; \\{{{\overset{\sim}{s}}_{i}(t)} = {{s_{i}(t)} - {\frac{H_{i}B_{i - 1}\sqrt{P_{i - 1}}}{H_{i}B_{i}\sqrt{P_{i}}}{{\overset{\sim}{s}}_{i - 1}(t)}}}} & (14)\end{matrix}$

Therefore, the nonlinear processing unit 18 corrects s(bold face)_(i)(t)according to the above Expression (13). The terminal 2-1 is not set asan IUI terminal. That is, IUI does not occur in a received signal of theterminal 2-1. Therefore, it is unnecessary to apply the correction tothe transmission signal s(bold)₁(t) to the terminal 2-1. Therefore, bysequentially determining transmission signals to set s(bold)₁(t) asknown, correct s(bold)₂(t), and correct s(bold)₃(t) using s(bold)₂(t)after the correction, it is possible to remove, on the transmissionside, that is, in the base station 1 c, in advance, IUI that occurs onthe reception side. That is, the nonlinear processing unit 18 is aninterference removing unit that performs sequential interference removalfor sequentially determining transmission signals from a transmissionsignal of the terminal 2 not set as an IUI terminal and removinginterference. In this embodiment, the processing for sequentiallydetermining transmission signals from a transmission signal of theterminal 2 not set as an IUI terminal and removing interference asexplained above is referred to as sequential interference removal. Inthe sequential interference removal, as it is seen from Expression (12),because IUI does not occur in the terminal 2 in a first place, which isthe next number of the last terminal, that is, the next place of an m-thplace, the nonlinear processing unit 18 sequentially carries out, up tothe m-th terminal, processing for removing interference that occurs inthe terminal 2 in the next place. By applying the system precodingmatrix in this embodiment, unlike the general nonlinear MU-MIMOprocessing in which the number of transmission interferencecancellations increases in proportion to the number of terminals, it ispossible to limit the number of interference cancellations to only thenumber of interference cancellations for one user. Therefore, comparedwith the general non-linear MU-MIMO processing, it is possible tosuppress deterioration due to a computation amount reduction and asignal subtraction.

The non-linear processing unit 18 applies the sequential interferenceremoval to transmission signals, thereafter multiplies the transmissionsignals with a power distribution matrix P(bold face)_(i) generated onthe basis of power distribution, further multiplies the transmissionsignals with the system precoding matrix B(bold face)(bar) calculated bythe processing explained above, and transmits the transmission signalsto the transmission-waveform shaping units 14-1 to 14-T.

However, an actually transmitted signal is expanded or reduced to beunstable because of transmission processing given by Expression (14).Therefore, the nonlinear processing unit 18 can apply processing forstabilizing a transmission signal waveform according to a modulooperation disclosed in “H. Harashima and H. Miyakawa,‘Matched-Transmission Technique for Channels With IntersymbolInterference,’ IEEE Trans. Commun., vol. 20, August 1972.” orperturbation processing disclosed in “B. M. Hochwald, C. B. Peel, and A.L. Swindlehurst, ‘A Vector-Perturbation Technique for Near-CapacityMultiantenna Multiuser Communication-Part II: Perturbation,’ IEEE Trans.Commun., vol. 53, no. 3, pp. 537-544, March 2005.”

A hardware configuration of the base station 1 c is explained. The samecomponents as the components in the first embodiment among components ofthe base station 1 c can be realized by the hardware configurationexplained in the first embodiment. The precoder unit 12 b and thenonlinear processing unit 18 in this embodiment are processing circuits.The precoder unit 12 b and the nonlinear processing unit 18 can bededicated hardware or can be a control circuit including a memory and aCPU that executes a program stored in the memory. The control circuitthat realizes the precoder unit 12 b and the nonlinear processing unit18 is, for example, the control circuit 400 shown in FIG. 5.

As explained above, in this embodiment, the IUI terminal is not set inone of the terminals 2, one IUI terminal is set in the other terminals 2as in the first embodiment, and the system precoding matrix isgenerated. Transmission signals are corrected to remove, on thetransmission side, in advance, interference that occurs on the receptionside by the nonlinear processing unit 18. Therefore, the same effects asthe effects in the first embodiment are obtained. It is possible tolayer a multiuser space and it is possible to realize a nonlinearMU-MIMO scheme in which deterioration due to a computation amountreduction and a signal subtraction is suppressed.

The configurations explained in the embodiments above indicate examplesof the contents of the present invention. The configurations can becombined with other publicly-known technologies. A part of theconfigurations can be omitted or changed in a range not departing fromthe spirit of the present invention.

REFERENCE SIGNS LIST

-   -   1, 1 a, 1 b-1 to 1 b-q, 1 c base station    -   2-1 to 2-m terminal    -   3 control station    -   11-1 to 11-m primary modulating unit    -   12, 12 a, 12 b precoder unit    -   13, 32 ordering unit    -   14-1 to 14-T, 14-c transmission-waveform shaping unit    -   15-1 to 15-T, 15-c, 21-1 to 21-R antenna    -   16 receiver    -   17, 33 transceiver    -   18 nonlinear processing unit    -   22-1 to 22-R reception-waveform shaping unit    -   23 decoder unit    -   24 demodulating unit    -   25 transmitter    -   31 precoder calculation unit.

1: A transmitting apparatus comprising: a plurality of transmissionantennas capable of forming a plurality of beams respectively directedto a plurality of receiving apparatuses; and a precoder to performprecoding on signals transmitted from the transmission antennas suchthat received power in third receiving apparatuses, which are thereceiving apparatuses excluding a first receiving apparatus serving as atransmission destination of a transmission signal among the receivingapparatuses and a second receiving apparatus, which is one of thereceiving apparatuses, is equal to or smaller than a threshold. 2: Thetransmitting apparatus according to claim 1, wherein received power inthe second receiving apparatuses is larger than the threshold. 3: Thetransmitting apparatus according to claim 1, wherein signals transmittedfrom the transmission antennas are multiplied with a first matrix, whichis a precoding matrix for reducing the received power in the thirdreceiving apparatuses to the threshold or less, and a multiplicationresult is multiplied with a second matrix, which is a precoding matrixfor forming a beam directed to the first receiving apparatus. 4: Thetransmitting apparatus according to claim 1, wherein the secondreceiving apparatus corresponding to the first receiving apparatus isselected on the basis of a correlation between a transmission linematrix between the first receiving apparatus and the transmittingapparatus and transmission line matrixes between the receivingapparatuses other than the first receiving apparatus and thetransmitting apparatus. 5: The transmitting apparatus according to claim1, wherein the second receiving apparatus corresponding to the firstreceiving apparatus is selected on the basis of geographical separationdegrees between the first receiving apparatus and the receivingapparatuses other than the first receiving apparatus. 6: Thetransmitting apparatus according to claim 1, further comprising anordering unit to determine order of the receiving apparatuses in theprecoding. 7: The transmitting apparatus according to claim 6, whereinthe ordering processor carries out power distribution to the receivingapparatuses, and the precoder multiplies signals transmitted from thetransmission antennas with a power distribution matrix corresponding toa result of the power distribution and a precoding matrix for carryingout the precoding. 8-9. (canceled) 10: The transmitting apparatusaccording to claim 6, wherein the order is order decided such that thereceiving apparatus next to the first receiving apparatus in order isthe second receiving apparatus corresponding to the first receivingapparatus. 11: The transmitting apparatus according to claim 10, whereinthe ordering processor performs the ordering such that the receivingapparatuses continuous in order are geographically close to or separatedfrom each other.
 12. (canceled) 13: The transmitting apparatus accordingto claim 10, further comprising an interference remover to performsequential interference removal, wherein a precoding matrix for settingthe receiving apparatus last in order among the receiving apparatusesordered by the ordering processor as the first receiving apparatus is amatrix for performing the precoding such that received power in fourthreceiving apparatuses, which are the receiving apparatuses excluding thefirst receiving apparatus, is equal to or smaller than the threshold anda precoding matrix for setting the receiving apparatus other than thereceiving apparatus last in order among the receiving apparatuses as thefirst receiving apparatus is a matrix for performing the precoding suchthat received power in the third receiving apparatuses is equal to orsmaller than the threshold, and the interference remover sequentiallycarries out, as the sequential interference removal, processing forsequentially removing interference that occurs in the receivingapparatuses next in order excluding interference removal processingcorresponding to the receiving apparatus in a first place, which is anext place of a last place.
 14. (canceled) 15: The transmittingapparatus according to claim 6, wherein the ordering processor sets theorder to descending order of non-negative eigenvalues or non-negativesingular values of transmission line matrixes between the transmittingapparatus and the receiving apparatuses. 16: The transmitting apparatusaccording to claim 6, wherein the ordering processor sets the order toascending order of non-negative eigenvalues or non-negative singularvalues of transmission line matrixes between the transmitting apparatusand the receiving apparatuses. 17: A receiving apparatus that receivessignals transmitted from the transmitting apparatus according to claim1, the receiving apparatus comprising a decoder to extract a desiredsignal from the signals received from the transmitting apparatus. 18-21.(canceled) 22: A control station in a communication system capable offorming a plurality of beams respectively directed to a plurality ofreceiving apparatuses by a plurality of transmission antennas mounted ona plurality of transmitting apparatuses, the control station comprising:a precoder calculator to calculate a precoding matrix for performingprecoding such that received power in third receiving apparatuses, whichare the receiving apparatuses excluding a first receiving apparatusserving as a transmission destination of a transmission signal among thereceiving apparatuses and a second receiving apparatus, which is one ofthe receiving apparatuses, is equal to or smaller than a threshold, anda transceiver to transmit the precoding matrix to the transmittingapparatuses. 23-24. (canceled) 25: A transmission precoding method in atransmitting apparatus including a plurality of transmission antennascapable of forming a plurality of beams respectively directed to aplurality of receiving apparatuses, the transmission precoding methodcomprising: a first step of determining a first receiving apparatus,which is the receiving apparatus serving as a transmission destinationof a transmission signal, and second receiving apparatuses, which is thereceiving apparatuses other than the first receiving apparatus; and asecond step of performing precoding on signals transmitted from thetransmission antennas such that received power in third receivingapparatuses, which are the receiving apparatuses excluding the firstreceiving apparatus and the second receiving apparatuses is equal to orsmaller than a threshold.